XDR... esoteriec microphone preamplifier sound quality
is now affordable.
Room ambiance so detailed you can practically hear the
carpet pattern... high frequency resolution that defines cymbals, triangles
and bells down to the molecular level... midrange that's as warm, fluid
and comfortable as a warm bath... and steel-fisted bass that's intoxicatingly
rich in harmonic texture.
Spending $1000 a channel for an "audiophile" mic preamplifier is worth
it... because no matter how much you spend on a microphone, its ultimate
performance depends on what it's plugged into and how the two interact.
Yes, many high-end outboard mic preamps can effortlessly amplify the slightest
sonic nuance. They can create an aural panorama that is breathtakingly
realistic, excitingly vivid, and truly 3-dimensional in scope. Although
extremely expensive, outboard microphone preamps have filled a need for
fidelity that wasn't possible with the "stock" mic preamps built into
mixing consoles. Until now.
No other company in the world has spent the sheer number of research hours
and $250,000 in R&D costs on a single mic preamp design that we just
did. Greg Mackie believed it was worth it because a new vastly-improved
design could be used on so many different mixer models – and thus
could be used by far more people than any expensive esoteric outboard preamp.
We started with blank paper, concerned only with matching or exceeding the
performance of $1000 to $2000-per-channel esoteric outboard preamps. By
the time we were finished, we ended up re-engineering far more than just
Mackie's core preamp technology.
1. A mic preamp should be able to protect itself
from destructive external forces.
2. A mic preamp should not be prone to Radio Frequency Interference
3. A mic preamp's response should not vary with different mic cable
4. A mic preamp should be accurate, sound good, and have very low
noise and wide bandwidth at any gain level.
Many high-end outboard mic preamps achieve Goals 3 & 4, but lack protection
and good RFI shielding. Conventional mixer preamps often meet Goals 1 and
2 at the expense of good sound1.
XDR represents major advances in all four critical areas.
Slow degradation (and eventual failure) of mic preamps resulting from hot
patching is one of the mixing console manufacturing community's dirty little
secrets. If you route a phantom-powered mic through a patchbay, it's roughly
equivalent to shorting out a cable every time you patch it. The mic preamp's
input transistors progressively break down (called zenering), while the
mixer channel gets noisy and can eventually totally croak.
Mackie engineers weren't interested in any "solution" that even slightly
affected the preamp's sound. A lot of research and considerable, old-fashioned
trial and error listening resulted in the addition of ultra-high-speed,
large-geometry input diodes to the front end of our input circuit. It completely
protects XDR mic preamplifiers from the consequences of hot-patching and
direct short circuits in cables carrying phantom power. Yet it has no affect
on sound quality.
A mic preamp has to be able to amplify faint one-millivolt2
input signals up to a thousandfold (60dB). In the process,
its rectification components can also pick up radio frequency interference
(RFI) from AM and FM stations, cell phones, and pager transmitters –
even microwave ovens – and amplify them to audible levels.
The more capable of conveying fine sonic details and wide bandwidth a mic
preamp is, the more prone it is to picking up the nearest trucker's CB or
All Sports Talk Radio station.
We assaulted RFI on three fronts. First, we incorporate bifilar wound DC
pulse transformers with high permeability cores that reject RFI but don't
attenuate the sound at 20kHz and above. Second, we carefully matched high-precision
components for critical areas of the XDR mic preamplifier. Third, we direct-coupled
the circuit from input to output and used pole-zero-cancellation constant-current
biasing (which also avoids increased intermodulation distortion at high
common mode signal levels).
Bottom line for the non-technical: you can use our new XDR mic preamps at
the end of extremely-long cheap mic cables in an RFI-saturated urban environment
while talking on your cell phone and microwaving coffee... without hearing
a trace of RFI.
A microphone's frequency response (and thus how it sounds) is a function
of the load presented to it. That load is the impedance characteristic of
the mic preamp it's connected to. If a mic preamp isn't designed right,
it will actually sound different depending on the impedance of the microphone
and the cable load3!
What we've done with the XDR design is create a Controlled
Interface Input Impedance system that's able to accept an enormous range
of impedances without compromising frequency response. The graphs at left
show XDR frequency response with 50-ohm, 150-ohm and 600-ohm loads. Note
that the charts are calibrated in TENTHS of a dB. As you can see, at any
load, frequency response is virtually unaffected down to just one tenth
of a dB at 20Hz and 20kHz no matter what the load.
We call our new design "XDR" because it has over 130dB of total dynamic
range. That means the XDR mic preamps on our VLZ Pro mixers can handle inputs
from 24-bit, 192kHz sampling-rate digital audio workstations without added
coloration. It means you can generate 145dB sound pressure levels at the
diaphragm of a suitable microphone and get a signal that doesn't break up.
But wide dynamic range is valuable only if the noise floor stays low at
normal gain levels.
Equivalent Input Noise (E.I.N.) is a common mic preamp noise spec. But this
oft-touted specification is traditionally measured at maximum gain levels.
Normal operating levels are most often between +20 and +30dB of gain. Thus
many mic preamps that brag about their -129dBm E.I.N. at +60dB generate
far more noise at +30dB where you're going to end up using them much of
The graph at right charts E.I.N. versus gain level for
three mic preamps that boast -129dBm at +60dB. The measurements represent:
a) a high-profile competitor's mic/line mixer preamps; b) our new XDR design
and; c) a "status" outboard mic preamp retailing for about $2,000. As you
can see, our XDR design maintains lower noise levels in the critical +20
to +30 range than either competitor. XDR mic preamps can handle input signals
from a full +22dB line level (thanks to extended dynamic range) down to
microvolts without a corresponding increase in noise.
While specifications are only one measure of how a microphone preamplifier
sounds, XDR mic preamps match esoteric outboard designs number for number:
0.0007% Total Harmonic Distortion. A THD figure this low has never
before been offered on affordable mic/line mixers (and only rarely on $500,000
mega consoles, for that matter).
Near "DC-to-Light" bandwidth – without RFI side-effects. Not
only are XDR mic preamps flat within a tenth of a dB across the bandwidth
of any known microphone, but they're also only 3dB down at an astonishing
192kHz, a figure that should satisfy anyone in search of elusive upper harmonics
soon available with emerging ultra-high sampling rates.
Super-low intermodulation distortion at high operating levels. Or
to put it another way... total independence and stable operation no matter
what signal level you send it. This is due to our instrumentation-style
balanced differential architecture, linear biasing, and use of DC-coupled
pole-zero-cancellation constant current that frees the mic preamp from induced
dynamic intermodulation distortion.
We can sling Audio Precision® graphs around 'till the cows come home
and still not convince the truly critical listener. When it comes to mic
preamps, the proof is in the listening. Plug a really, really good condenser
microphone into an XDR mic preamp and do some critical auditioning.
Notice the highly-focused high end. The superior ambience retrieval. The
substantial, detailed lower bass octaves. Hear textural midrange details
you never knew the microphone could reproduce.
Above all, hear this kind of mic preamplifier performance on truly affordable
mic/line mixers for the very first time.